Fibre Channel is a computer communications protocol designed to meet the many requirements related to the ever increasing demand for high performance information transfer. The Fibre Channel protocol is sometimes referred to in the literature as Fiber Channel; the variation is due to differences in spelling between American English and British English. Fibre Channel combines the benefits of both channel and network technology and also provides for flexible topologies, connectivity over several kilometers (not to exceed generally 10 kilometers (km)), and support for multiple relatively high data rates, media types, and connectors. Fibre Channel has become relatively popular for connecting multiple storage devices together to form a storage area network (SAN). The popularity is partly due to the fact that once a channel between two devices is set up very little decision making is needed, allowing for a high speed, hardware intensive environment.
Increasingly, however, SANs are becoming geographically dispersed. This dispersion is due to many factors: mergers and acquisitions of companies located nationally and internationally; desire to provide off-site storage; and storage replication, among other factors. Due to the upper limit on the distance of about 10 km between two devices on a Fibre Channel network, it is impracticable to interconnect two storage area networks that are separated by a distance greater than 10 km using Fibre Channel.
To allow communication between two FC enabled SANs, SANs have been interconnected by a non-Fibre Channel network or fabric (or point-to-point interconnect) that supports communication over large distances (greater than 10 km). Unfortunately, a non-FC network or interconnect may introduce a substantial delay, leading to a throughput between the two SANs that is less than the actual capacity or bandwidth of the two SANs.
FIG. 1 illustrates a block diagram of an inter-network including two FC enabled devices, and a network or fabric. FC devices communicate by sending RRDY signals to each other which indicate that one device is ready to receive a frame of data from the other device. A RRDY has to be received by the other device before the other device can transmit a frame of data. For purposes of illustration, device 110 and device 130 have a communication link or channel set up between them which is ready for the transfer of data from device 130 to device 110. Device 110 sends a RRDY to device 130. Since devices 110 and 130 are coupled by network 210, which causes a delay, the RRDY arrives at device 130 at a time Δt after it was transmitted. Device 130 sends a frame of data to device 110 after it receives the RRDY. It also takes a time of Δt for the frame to arrive at device 110. If there are N bits in the frame, the effective transmission rate for the frame is N/(2Δt). Assuming that Δt is larger than the time it takes to transmit the bits of a frame (δt), the effective transmission rate or throughput of communications link is lower than the actual generation or transmission rate of the frame (N/δt). The frame of data, even though it was generated at a relatively high data rate (actual transmission rate), has an effective transmission rate that is lower than the actual transmission rate.
Several solutions have been proposed for increasing the effective transmission rate. However, these solutions may cause dropped frames due to excessive data flow. Discarded or dropped frames have to be sent again which means that the effective bandwidth is less than what it could be if frames were not discarded or dropped.
As described above, existing solutions are not capable of providing efficient communication between geographically dispersed SANs. Consequently, it is desirable to interconnect geographically dispersed Fibre Channel enabled SANs in a manner that allows efficient communication.